Video surveillance cameras are increasingly used in many applications, such as security and traffic monitoring. In most applications, analog cameras are typically employed. Analog cameras produce uncompressed video images. Digital cameras, on the other hand, generate compressed signals that use, for example, a Motion JPEG (M-JPEG) or H.264 coding standard. With the Motion JPEG coding standard, for example, each video frame or interlaced field of a digital video sequence is separately compressed as a JPEG image.
Surveillance applications typically must store the captured video images. Due to the large number of video images to be stored, the storage cost becomes an important consideration. Thus, an efficient video compression scheme is an important component of any video storage system. With the Motion JPEG coding standard, each image is coded independently of other images, using a technique referred to as intra-frame coding. With intra-frame coding, compression techniques are performed relative to the information contained only within the current frame (and not relative to any other frame in the video sequence). Intra-frame coding provides easy post-processing, such as editing of the images, because any image in the video scene may be directly accessed. Intra-frame coding, however, uses more bits than predictive coding or inter-coding.
With inter-frame coding, such as MPEG coding, a video compression stream is expressed in terms of one or more neighboring frames. Inter-frame prediction leverages temporal redundancy between neighboring frames and thereby provides higher compression rates. Existing video coding techniques, such as MPEG and H.264, were developed primarily for broadcasting and DVD applications and employ well-known Group of Picture (GOP) techniques.
GOP techniques specify a maximum number of frames between each intra-frame, such as every 15 frames. The intra-coded frames provide the required entry points for prompt channel changes in broadcasting applications and fast forward and backward searching in DVD applications. The maximum number of frames between each intra-frame requires existing video coding techniques to periodically encode an intra-coded frame, without regard to whether the level of activity in the monitored scenes even requires an intra-frame. Thus, even with inter-frame coding, the high temporal correlation in the consecutive pictures is not fully exploited, resulting in lower compression performance.
In many applications, however, including surveillance applications, the monitored scenes often have little activity. A need therefore exists for improved techniques for coding and compression of surveillance video, and video from low activity applications that do not necessarily require fast editing and channel switching. A further need exists for improved techniques for video coding that do not require fixed or adaptive GOP structures having a maximum number of consecutive inter-coded frames.